Digital synchronizing and phasing system

ABSTRACT

Digital systems for synchronizing prime movers equipped with magnetic ball head speed governors provide smooth and continuous synchronizing control. A plurality of prime movers may be synchronized by operating one as a &#39;&#39;&#39;&#39;master&#39;&#39;&#39;&#39; and the other as &#39;&#39;&#39;&#39;slaves,&#39;&#39;&#39;&#39; in which event each &#39;&#39;&#39;&#39;slave&#39;&#39;&#39;&#39; is equipped with a magnetic ball head speed governor and the bias coil of each such governor is supplied with a digital control signal which is modulated in accordance with any difference between the speed of the associated &#39;&#39;&#39;&#39;slave&#39;&#39;&#39;&#39; and that of the &#39;&#39;&#39;&#39;master.&#39;&#39;&#39;&#39; Alternatively, a pair of prime movers may be synchronized by equipping each of them with a magnetic ball head speed governor and by supplying the bias coils of those governors with digital signals which are complementarily modulated in accordance with any difference between the speeds of the prime movers. The reference level of the digital control pulses may be adjusted manually or automatically to establish and maintain a desired phase relationship between the synchronized prime movers.

United States Patent Leeson, J r.

[ Jan. 15, 1974 DIGITAL SYNCHRONIZING AND PHASING SYSTEM [75] Inventor: James L. Leeson, Jr., Rockford, Ill.

[73] Assignee: Woodward Governor Company, Rockford, Ill.

[22] Filed: June 14, 1972 [2]] Appl. No.: 262,792

[52] US. Cl. 60/97 S, 60/105, 318/318 [51] Int. Cl. F0lb 25/06 [58] Field of Search 60/97 S, 105;

[56] References Cited UNITED STATES PATENTS 3,097488 7/1963 Eggenberger et al 60/105 X 3,367,110 2/1968 Leeson, Jr. 60/97 S 3,408,549 10/1968 Shimabukuro 318/318 X 3,689,175 9/1970 Hartzell et al.... 6 0/97 S 3,546,553 12/1970 Loyd 318/318 3,643,437 2/1972 Birnbaum et a1. 450/10 Primary ExaminerMartin P. Schwadron Assistant Examiner-Allen M. Ostrager AttorneyWolfe, Hubbard, Leydig, Voit & Osann [57] ABSTRACT Digital systems for synchronizing prime movers equipped with magnetic ball head speed governors provide smooth and continuous synchronizing control. A plurality of prime movers may be synchronized by operating one as a master" and the other as slaves, in which event each slave is equipped with a magnetic ball head speed governor and the bias coil of each such governor is supplied with a digital control signal which is modulated in accordance with any difference between the speed of the associated slave and that of the master." Alternatively, a pair of prime movers may be synchronized by equipping each of them with a magnetic ball head speed governor and by supplying the bias coils of those governors with digital signals which are complementarily modulated in accordance with any difference between the speeds of the primemovers. The reference level of the digital control pulses may be adjusted manually or automatically to establish and maintain a desired phase relationship between the synchronized prime movers.

19 Claims, 11 Drawing Figures DIGITAL SYNCIIRONIZING AND PHASING SYSTEM BACKGROUND OF THE INVENTION The present invention relates generally to digital synchronizing systems and, more particularly, to digital synchronizing systems for prime movers. One aspect of the invention also pertains to phase control systems for prime movers.

One of the more important, although by no means exclusive, applications for synchronizing systems is in synchronizing two or more prime movers so that they drive their respective loads at substantially identical speeds. For example, it is normally desirable in twin engine, propeller driven aircraft to have the propellers driven at identical speeds since otherwise there are undesirable beats," throbbing and vibrations which are not only disturbing to the occupants but also poten-v tially damaging to the aircraft structure. Gross speed differences between the propellers may be readily eliminated manually, and toward that end the manual speed control throttles are customarily mounted in close proximity with one another to facilitate their corresponding and simultaneous manipulation by the pilot. Precise engine synchronism is, however, virtually impossible to achieve manually. There is almost always some minor difference in throttle settings, throttle linkages and/or engine loads; any of which will tend to cause a relatively small, but still significant, difference between the engine speeds. Thus, to eliminate such speed differences, it is conventional to include an automatic synchronizing system.

In some instances, very close, or indeed even precise, speed synchronization does not provide a full solution to the engine control problem. The engines of, say, a twin engine, propeller driven aircraft may be precisely synchronized and, yet, undesirable vibration may still be encountered. Typically, such vibration is caused by a lack of perfect balance in the engine crankshafts and other rotating parts such that its magnitude depends on the relative phase angle between the engines. Hence, a

phase controller is also sometimes included for adjusting the phaserelationship of the synchronized engines to thereby permit the amount of vibration to be minimized.

Digital systems for synchronizing prime movers are available in the prior art. Electrically, such systems have the distinct advantage over analog types of being substantially immune to electrical variations, such as power supply drift, sensitivity drift and component aging, and to environmental variations, such as ambient temperature changes. This advantage flows from the fact hat digital s mate qat th messa e g ab sence of signals-Le, the high (1) and low levels of the signals rather than to signals of continuously varying amplitude. Synchronization of prime movers is, however, essentially a continuous control problem, and known digital synchronizing systems have, as a general rule, fallen short of providing the smooth, continuous control required to most effectively solve the control problem. Specifically, prior art digital synchronizing systems have conventionally relied on control elements having discrete output responses, such as stepping motors, for carrying out the corrective speed adjustments required for speed synchronizing. Thus, such systems have not only failed to provide smooth, continuous synchronizing control, but

they have also often required undesirably complex mechanical linkages for translating the discrete output responses of the control elements into effective corrective action for the engines.

SUMMARY OF THE INVENTION Accordingly, a primary aim of the present invention is to provide a digital system for synchronizing prime movers which not only provides relatively smooth, continuous control, but which also eliminates the need for mechanical linkages between the control system and the prime mover. It has been found that very effective synchronizing control of prime movers equipped with magnetic ball head speed governors may be obtained by applying digital control signals which are modulated in accordance with the speed error or errors that are to be eliminated to the bias coils of the governors. A magnetic ball head speed governor has inherent mechanical and electrical inertia which may be advantageously em ployed to average such a control signal, thereby enabling the governor to smoothly and continuously correctively adjust the speed of its associated prime mover.

A plurality of prime movers may be synchronized in accordance with this invention by operating one as a master" and the others as slaves, in which event each slave engine or prime mover is equipped with a magnetic ball head speed governor and the bias coil of each such governor is supplied with a digital signal which is modulated in accordance with any difference between the speed of its associated engine and the master" engine. Alternatively, if there are only two prime movers to be synchronized, each may be equipped with a respective magnetic ball head speed governor and the synchronization may be carried out by supplying the governor coils with respective digital signals which are complementarily modulated in accordancewith any speed difference. The magnetic ball head speed governors may be polarized, by which it is meant that their flyweights are permanent magnets, or non-polarized, by which it is meant that their fly weights are comprised of magnetic material but not permanently magnetized.

Another object of this invention is to provide relatively sensitive and fast acting digital synchronizing systems for prime movers and yet wherein the hardware 7 which comprises the systems is extremely simple, compact and low in cost. Because the synchronizing control in these systems is continuous, there is no appreciable dead band. Also, the averaging of the digital signals provides an oscillating component or dither signals which significantly reduces the adverse effects of friction within the associated governor.

A further object is to provide manual and automatic phase control systems which may be included with digital systems suitable for synchronizing prime movers in accordance with this invention.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of this invention will become apparent as the following detailed description is read in conjunction'with the attached drawings, in which:

FIG. 1 is a simplified illustration, partly in schematic and partly in block diagram form, of an embodiment of this invention which is suitable for synchronizing a slave engine having a non-polarized magnetic ball head speed governor with a master engine;

FIG. 2 is a simplified, fragmentary, sectional view of a magnetic ball head speed governor which may be used in implementing the various embodiments of the present invention;

FIG. 3 is a timing diagram showing typical waveforms of signals occurring in the embodiment of FIG. 1 during the synchronizing of a relatively slow slave engine;

FIG. 4 is a timing diagram similar to that of FIG. 3, but showing typical waveforms of signals occurring in the embodiment of FIG. 1 during the synchronizing of a relatively fast slave engine;

FIG. 5 is a fragmentary illustration, partly in schematic and partly in block diagram form, of another embodiment of this invention which is suitable for synchronizing a slave engine having a polarized magnetic ball head speed governor with a master engine;

FIG. 6 is a timing diagram showing typical waveforms of signals occurring in the embodiment of FIG. 5 during the synchronizing of a relatively slow slave engine;

FIG. 7 is a fragmentary, schematic diagram of a combined synchronizer and manual phase controller for use with a slave engine equipped with a polarized magnetic ball head speed governor;

FIG. 8 is a fragmentary, schematic diagram ofa combined synchronizer and automatic phase controller also for use with a slave engine equipped with a polarized magnetic ball head speed governor;

FIG. 9 is a timing diagram showing the waveforms of signals occurring in the embodiment of FIG. 8 during the synchronizing and phasing ofa relatively slow slave engine;

FIG. 10 is a fragmentary illustration, partly in schematic and partly in block diagram form, of still another embodiment of the present invention which is suitable for synchronizing a pair of engines equipped with respective non-polarized magnetic ball head speed governors; and

FIG/11 is a timing diagram showing typical waveforms occurring in the embodiment of FIG. 10 during the synchronizing of the engines.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT While the invention is described in some detail hereinafter with reference to certain illustrated embodiments, it will be understood that the intent is not to limit it to such detail. On the contrary, the intent is to cover all modifications, alternatives and equivalents falling within the spirit and scope of the invention as defined by the appended claims.

A. Introduction For exemplary purposes, the description which follows is directed toward synchronizing and phasing of a pair of prime movers, such as the engines of a twin engine, propeller driven aircraft. It will, however, be appreciated that those embodiments of the invention which operate on a master-slave principle are of broader utility since additional prime movers may readily be accommodated simply by operating them as further slaves, and in some applications the speed of a single engine may be controlled to match the frequency of a changeable frequency recurring signal which comes from some source other than a master prime mover.

Turning now to the drawings, and particularly to FIG. 1, there are two rotating prime movers or engines 2l and 22 with their output drive shafts 23 and 24 coupled to drive respective loads (not shown). The engines are equipped with respective governors, and the speed setting arms 25 and 26 of the governors are coupled by flexible linkages 27 and 28 to respective manually adjustable levers 30 and 31. Thus, the speeds of the engines may be adjusted separately or in unison by shifting the positions of the levers 30 and 31. This permits the engines to be brought to roughly the same speed before the automatic synchronizing process begins.

Automatic synchronizing is carried out in accordance with the invention by equipping one or more of the engines with magnetic ball head speed governors and by applying to the coil of each such governor a digital signal which is modulated according to any existing speed error. Thus, it will be helpful to briefly review the contruction and operation of a suitable magnet ball head speed governor.

B. A Magnetic Ball Head Speed Governor The null or balanced speed of a magnetic ball head speed governor depends jointly on the spring bias and the magnetic bias applied to its flyweights. Such governors have heretofore been used for prime movers calling for very accurate set point speed adjustment. Specifically, the spring bias has been varied for coarse speed adjustments, and the magnetic bias has been varied for finer, Vernier-like trimming" adjustment of the set point speed for the associated prime mover. Customarily, both the coarse and fine adjustments have been carried out manually. In keeping with this invention, however, it has been found that such governors are capable of providing relatively rapid, dynamic speed control for effective automatic speed synchronization as a master speed or a reference frequency vary over a wide range.

More particularly, a magnetic ball head speed governor is advantageously employed in digital synchronizing and phasing systems of the present invention.

A preferred magnetic ball head speed governor is the one described and claimed in the copending, commonly assigned Davis application Ser. No. 262705, filed June 14, 1972, on Magnetic Ball Head Speed Governor. As shown in FIG. 2 hereof, an exemplary embodiment of the preferred governor comprises a hollow casing 41 housing a ball head assembly 42 which includes a pair of diametrically opposed, pivotally mounted flyweights 43 and 44. The fuel controller (not shown) of the prime mover or engine to be regulated by the governor typically includes a speed controlling servo (also not shown) which is hydraulically controlled by a valve shown generally and in fragmentary form at 45. The valve may suitably comprise a spool or plunger 46 journaled in a sleeve 47 which has a lower extension (not shown) rotationally driven by the controlled engine, all as more fully described in the commonly assigned Drake U.S. Pat. No. 3,251,373 entitled All Speed Balanced Governor. It sufflces here to note that the ball head assembly 42 is mounted for rotation with the valve sleeve 47 and that the upper end of the valve plunger extends into the ball head casing 41 where it terminates in an annular flange 48. As will be seen, the flange 48 rides on an anti-friction bearing 50 which overlies a washer 51 which, in turn, bears on the toes of the flyweights 43 and 44 inwardly of their respective pivot points 52 and 53. Consequently, the

valve plunger 46 is shifted axially of the sleeve 47 as the flyweights rotate about their pivot points.

The spring bias for the illustrated governor is supplied by a speeder spring 49 which bears at its lower end against the flange 48, thereby tending to cause the flyweights 43 and 44 to pivot inwardly. The upper end of the speeder spring 49 abuts aganist an annular collar 54 which is mounted on a shaft 55 that is threaded into a tubular extension of a cover 56 for the ball head casing. The stressing of the speeder spring 49 is manually adjustable by means of a speed setting arm 57 which is fixed to the upper end of the shaft 55.

The magnetic bias, on the other hand, is supplied by a magnetic field which is created in response to current flow through a coil 57. In this instance, the coil 57 is spaced radially inwardly of the flyweights 43 and 44. The flyweights must, of course, be magnetically responsive. To that end, in a polarized" governor the flyweights 43 and 44 include permanent magnets so that they are either attracted toward or repelled from the coil 57 in dependence on the direction of current flow through the coil. In contrast, in a non-polarized governor the flyweights are comprised of magnetic material, say, soft iron, but are not magnetized, with the result that they are attracted toward the coil regardless of the direction of current flow. The magnitude of the magnetic bias acting on the flyweights of either type of governor depends on the magnetic field intensity which, in turn, depends on the magnitude of the current flow through the coil. In operation of the magnetic ball head speed governor, rotation of the controlled engine at its set point speed causes the ball head assembly 42 to be rotated at precisely the speed necessary to enable the flyweights to develop a centrifugal force equal and opposite to the net bias (the algebraic summation of the spring bias and the magnetic bias) applied to the flyweights 43 and 44. The governor is then balanced, meaning that the flyweights are held in predetermined null positions, say, substantially parallel to the vertical axis of the governor, thereby causing the valve plunger 47 to be centered axially within the sleeve 46. If, however, the engine is operating at a speed above or below its set point speed, there is an unbalanced condition which causes the flyweights to rotate outwardly or inwardly, thereby causing the valve plunger to shift upwardly or downwardly relative to the sleeve so as to slow down or speed up the controlled engine.

As will be seen, advantage is taken of the inherent electrical and mechanical inertia of the magnetic ball head speed governor in carrying out the present invention. Specifically, it has been found that the inductance of the magnetic biasing coil of such a governor, together with the mechanical inertia of the governor, averages digital control signals which are in the form of square wave pulses so that, in effect, an adjustable, steady magnetic force is applied to the flyweights, to establish the governor set point, according to the duty cycle or average value of such pulses. The averaging is performed with a time constant which is principally determined by the coil inductance and mechanical inertia of the movable parts, both of which permit some freedom of selection. Specifically, to the extent possible, that time constant should be selected such that the averaging of the digital control signal which is supplied when the controlled engine is at or near its synchronized speed results in a coil current characterized by having a small oscillating component superimposed on a dc. component which varies according to the duty cycle of an applied squarewave voltage. The dc. component is, therefore, effective to adjust the magnetic bias acting on the flyweights so as to smoothly and con tinuously correctively adjust the set point speed of the controlled engine, whereas the oscillating component serves as a dither signal to reduce the effects of friction within the governor and thereby decrease its response time. C. Master-Slave Synchronizing With a Non- Polarized" Magnetic Ball Head Speed Governor Returning with the foregoing in mind in FIG. 1, it will be seen that in this embodiment the engine 21 is operated as a master or speed reference inasmuch as its speed determines the synchronized speed for the other or slave engine 22. The master engine 21 may be equipped with any conventional governor (not shown) capable of maintaining its speed at the level dictated by the position ofits control lever 30. The slave engine 22, on the other hand, is equipped with amagnetic ball head speed governor which, in this case, is of the nonpolarized type.

The speeds of the master and slave engines are first roughly balanced by setting their respective control levers 30 and 31. During that preliminary period, a midrange value of exciting current is supplied to the magnetic biasing coil 57 of the slave engine governor so that the automatic phase of the synchronizing process may be carried out by increasing or decreasing the magnetic bias on the flyweights as necessary to bring the slave engine into precise synchronism with the master. Thus, as shown, the governor coil 57 has its upper end connected to a suitable supply source and its lower end connected in series with a switch 57a. Initially, while the speeds of the engines 21 and 22 are being manually adjusted after start-up, the switch 57a is positioned to return the governor coil 57 to ground through a resistor 58. The value of the resistor 58 is selected to cause the coil 57 to draw the current necessary to pro vide a suitable magneticbias reference level for the governor, such as, say, a dc. current equal tothe average current supplied to the coil 57 when a squarewave excitation (to be described below) has a 50 percent duty ratio, i.e., its on time equals half of the period of each recurring cycle. Then, when a rough manual speed balance has been achieved as indicated, for example, by tachometer (not shown) readings, the switch 57a is thrown to its alternative or automatic synchronizing position in which the governor coil 57 is returned to ground through the collector-emitter circuit of a transistor 59.

Automatic synchronizing of the slave engine 22 with the master engine 21 is carried out in keeping with this invention by increasing or decreasing the magnetic bias on the slave engine governor by means of a digital contional to the engine speed. In the illustrated embodiment, the transducers 61 and 62 are permanent magnet alternators which respectively comprise toothed wheels 63 and 64 and cooperating pick-up coils 65 and 66. The wheels are mounted on shafts 71 and 72 which are respectively coupled by belt and pulley mechanisms 73 and 74 to be driven by the master and slave engines 21 and 23. Thus, as the wheels are rotated, their teeth 67 and 68 are periodically brought into proximity with their pick-up coils, with the result that the magnetic reluctance between each wheel and cooperating pickup coil is periodically varied thereby causing generally sinusoidal signals to be induced into the pick-up coils. The frequencies of such sinusoidal signals are respectively proportional to the rotational speeds of the master and slave engines. Preferably, there is only one tooth per wheel so that the risk of a phase rollover (i.e., the same transducer providing two successive signals without an intervening signal from the other transducer) occurring is minimized.

The waveforms of the output signals are not ideal for generating the required control signal. Accordingly, the transducer outputs are coupled to respective inputs of a bistable circuit 79 by means of respective pulse shapers 75 and 76 and differentiators 77 and 78. The pulse shapers 75 and 76 are typically Schmitt triggers for converting the output signals from the transducers into corresponding pulse trains. Further, the bistable circuit 79 is typically a flip-flop circuit which responds to the positive-going transitions of its input signals. Thus, the differentiators 77 and 78 are included to provide very fast rise time, positive going, spike-like triggering pulses for the flip-flop 79 in response to the leading edges of the pulses supplied by the pulse shapers 75 and 76. None of the essential speed information is lost in this conversion process since the repetition rates of the triggering pulses from the differentiators 77 and 78 are respectively identical to the repetition rates of the pulse trains from the pulse shapers 75 and 76 which, in turn, are respectively identical to the frequencies of the ac. signals supplied by the transducers 61 and 62.

In a broad sense, the pulses derived from the pick-up coil may be viewed as a variable frequency reference signal which designates the desired speed at which the slaveengine is to run. Thus, the invention in its broader aspects may be practiced when these reference pulses come from any suitable source, e.g., from an adjustable frequency oscillator, which commands the desired speed of the engine 22.

As here illustrated, the triggering pulses from the differentiators 77 and 78 are respectively applied to the set S and reset R inputs of the flip-flop 79. Thus, unless phase rollover" occurs, the flip-flop 79 is alternately set by a triggering pulse from the differentiator 77 and then reset by a triggering pulse from the differentiator 78. The set output Q of the flip-flop 79 is coupled via a current limiting resistor 82 to the base of the transistor 59. Its reset output Q, on the other hand, is not used. The output signal appearing at the set output Q of the flip-flop 79 is at a high (I) logic level when the flip-flop is set and a low logic level when the flipflop is reset. Thus, the transistor 59 is switched into and out of conduction as the flip-flop 79 is set and reset. Preferably, the base-emitter drive current supplied by the flip-flop 79 when it is in its set state is sufficient to drive the transistor into saturation so that the collectoremitter current drawn through the coil 57 is substantially independent of external influences, such as ambient temperature changes.

Inspection of FIGS. 3 and 4 confirms that the digital signal appearing at the set output Q of the flip-flop 79 has its duty ratio modulated in accordance with any difference between the speeds of the master and slave engines. For example, if the speed of the slave engine 22 is slow relative to that of the master engine 21, the duty ratio of the digital control signal will progressively increase as shown in FIG. 3 to thereby increase the average or dc. component of the current flow through the governor coil 57. As a result, the magnetic bias on the governor flyweights is increased which, in turn, causes the speed of the slave engine to increaseuntil the slave speed equals the master speed, the frequency of the pulses from pick-ups 65 and 66 are equal, and the duty cycle remains fixed. On the other hand, if the slave engine is relatively fast, the duty ratio of the digital control signal is progressively reduced, thereby gradually decreasing the magnetic bias on the governor flyweights so as to decrease the slave engine speed. The corrective action in response to a speed mis-match is sufficiently fast and forceful that the speed of the slave engine 22 is corrected before phase roll-over of the pulses from pick-ups 65 and 66 can occur, i.e., before the duty ratio can reach 0 or I00 percent within a single cycle.

In passing it may be noted that during the first cycle or so of the digital control signal after the switch 57a is closed, the phase angle between the master and slave engines, rather than their relative speeds, principally determines the direction in which the magnetic bias on the governor is changed. That is, however, a transitory effect of relatively short duration which, for practical purposes, may be ignored. Even if the relative phase angles between the engines be such that the magnetic bias of the governor begins to change in the wrong direction as shown in FIGS. 3 and 4, it takes only a few cycles for the duty ratio modulation to become controlling. Hence, as indicated by the dot-dashed lines, the average or dc. component of the current flowing through the governor coil 57 quickly reaches the level at which the resulting magnetic bias on the flyweights correctively adjusts the speed of the slave engine so as to synchronize it with the master engine.

As previously mentioned, the ball head time constant is preferably selected so that a relatively small oscillating current or dither signal is superimposed on the dc. or average coil current when the speed of the slave engine is near the speed of the master. Even though the voltage applied to the coil 57 is a squarewave in form, the current rises and falls as shown in FIGS. 3 and 4 due to inductive lag. The oscillating current is helpful in aiding the governor to rapidly respond to any tendency of the slave engine speed to deviate once synchronism has been achieved. Specifically, the oscillating current component causes very slight, high frequency variations in the magnetic bias acting on the governor flyweights so as to reduce the undesirable effects of friction within the governor without materially affecting the slave engine speed.

As will be appreciated, if the magnetic ball head speed governor for the slave engine 22 has its bias coil spaced outwardly rather than inwardly of its flyweights, the same general synchronizing system may be employed, with the principal exception being that the complement of the described digital control signal would be required. Of course, the complement of the described control signal may be readily obtained, such as by driving the transistor 59 from the reset output Q of the flip-flop 79.

D. Master-Slave" Synchronizing With a Polarized Magnetic Ball Head Speed Governor Turning now to FIGS. and 6, one of the alternatives to the above described synchronizing system is to equip the slave engine 22 with a polarized magnetic ball head speed governor. Because of its bidirectional magnetic biasing capability, such a governor has the advantage in master-slave synchronizing systems of being compatible with the use of zero as a magnetic bias reference level, with the result that there is little, if any, risk of reference level drift and no need for action by the operator to turn-on" the synchronizer. That is, the switch 57a and resistor 58 in FIG. 1 need not be used.

To carry out the present invention in the embodiment of FIG. 5, digital control signals, which have their duty ratios complementarily modulated in accordance with any difference between the speeds of the master and slave engines, are applied to the opposite ends of the magnetic biasing coil 100 of the polarized governor. The electrical and mechanical inertia of the governor averages the digital control signals in phase opposition to another such that opposing bucking currents tend to flow through the coil to vary the net coil current in a sense dependent upon the sense of any speed difference. The magnetic bias acting on the governor flyweights is again varied to smoothly and continuously adjust the slave engine speed so as to synchronize it with the master engine.

More particularly, substantially the same means as previously described with reference to FIG. 1 may be employed to provide the digital control signals, with the sole exception being that one of the control signals is taken from the set output Q of the flip-flop 101, while the other control signal is taken from its reset output Q. The digital signals appearing at the set and reset out puts have their duty ratios complementarily modulated in accordance with any speed difference between the master and slave engines 21 and 22, providing that phase rollover is avoided. Specifically, the flip-flop 101 is alternately set and reset by triggering pulses from the differentiators 77 and 78, respectively. Further, the repetition rates of the triggering pulses or, more specifically, the set and reset pulses fromthe differentiators 77 and 78 are respectively proportional to the speeds of the master and slave engines. Hence, if there is any difference between the speeds of those engines, the time sequencing of the set and reset pulses changes in dependence on the sense and magnitude of the speed difference which, in turn, causes the duty ratios of the digital signals at the set and reset outputs of the flipflop 101 to change in dependence on the sense and magnitude of the difference. For example, if, as shown in FIG. 6, the slave engine is slow relative to the master engine, the reset pulses from the differentiator 78 occur less frequently than the set pulses from the differentiator 79 and, therefore, tend to leg increasingly further behind the set pulses. Thus, the flip-flop 101 resides in its set state for increasing periods of time and in its reset state for decreasing periods of time. Once synchronism is achieved, however, the set and reset pulses occur at the same rates and, consequently, the duty ratiosof the signals appearing at the set and reset outputs of the flip-flop 101 then stabilize. Of course,

when the flip-flop is in its set state, its set output 0 is at a high (1") logic level and its reset output 0 is at a low (0) logic level. Contrariwise, when the flip-flop is in its reset state, its set output Q is low (0) and its reset output Q is high (1). Thus, the duty ratios of the digital signals appearing at the set and reset outputs of the flipflop 101 are oppositely varied (i.e., complementarily modulated) as a function of any difference between the speeds of the master and slave engines.

As shown, the digital control signals provided by the flip-flop 101 are applied to the opposite ends of the magnetic biasing coil of the slave engine speed governor by a bridge circuit 102. To that end, the bridge circuit comprises a pair of input transistors 103 and 104 which have their bases respectively coupled by. current limiting resistors 115 and 116 to the set and reset output Q and Q of the flip-flop 101. The input transistors 103 and 104 are, in turn, coupled to control respective pairs of complementary current steering transistors 105, 106 and 107, 108 in diagonally opposite arms of the bridge, and the governor bias coil 100 is coupled across the output of the bridge. More specifically, the collectors of the input transistors 103 and 104 are coupled by respective collector load resistors 111 and 112 to a suitable supply source and their emitters are returned to ground by respective emitter load resistors 113 and 114. Further, the transistors 103 and 104 have their collectors respectively coupled by current limiting resistors 115 and 116 to the bases of the transistors and 107 and their emitters respectively coupled to the bases of the transistors 106 and 108. The emitters of the transistors 105 and 107 are connected to the supply source, and the emitters of the transistors 106 and 108 are returned to ground. Finally, the collectors of the transistors 105 and 107 are respectively coupled by resistors 113 and 114 to the collectors of the transistors 108 and 106 which, in turn, are coupled to the opposite ends of the governor biasing coil 100.

Now, if the flip-flop 101 is switched to its set state, base-emitter drive current is supplied for the transistor 103 causing it to switch to its conductive state. When that occurs, the collector-emitter circuit of the transistor 103 draws current through the base-emitter junction of the transistor 105 and drives current through the base-emitter junction of the transistor 106 such that the transistors 105 and 106 also switch into conduction. At the same time, however, the transistors 107 and 108 are held in their non-conductive states because the transistor 104 is in its non-conductive state. Hence, when the flip-flop 101 is switched to its set state, current flows through the collector-emitter circuit of the transistor 105, the load resistor 113, the bias coil 100 and the collector-emitter circuit of the transistor 106. As will.be seen, the current tends to flow through the coil 100 in a left-to-right direction or, say, with a positive sense.

0n the other hand, when the flip-flop 100 is switched to its reset state, precisely the converse takes place. The transistors 103, 105 and 106 are switched to their non-conductive states, while the transistors 104, 107 and 108 areswitched into conduction. Consequently, current then tends to flow through the governor coil 100 in a right-to-left direction or with a negative sense. The average or net direct current effect of alternately driving current in opposite directions through the coil 100 may be visualized by considering the situation where the pulses from differentiators 77 and 78 are equal in frequency and 180 out of phase. In that case equal currents will flow oppositely through the coil during successive equal times, and their average value will be zero, so that the set point of the slave governor is determined solely by its speeder spring. If the slave engine speed increases or decreases relative to the master, the periods of forward and reverse" current will change, thereby changing the average current value and automatically causing the magnetic bias on the slave governor flyweights to oppose or aid the speeder spring bias to establish a new set point force which brings the slave speed into equality with the master speed.

As will be appreciated from the foregoing, the digital control signals from the flip-flop 101 are averaged by the electrical and mechanical inertia of the ball head in phase opposition or bucking relationship with one another. Thus, the average current flow through the biasing coil 100 changes in a sense dependent on the sense of any speed difference between the master and slave engines and, in turn, varies the magnetic bias acting on the governor flyweights until the speed of the slave engine is correctively adjusted to eliminate the speed difference. For example, if the slave engine is slow relative to the master as shown in FIG. 6, there is a gradually increasing net positive current flow through the coil which increases the positive magnetic bias on the governor flyweights to thereby increase the effective speed setting and thus the speed of the slave engine. Again, the time constant of the ball head is preferably selected so that the net coil current comprises an average or dc. component, which regulates the slave engine speed, and a superimposed relatively high frequency component, which serves as a dither signal for the governor. E. Digital Synchronizing With Manual Phasing As thus far described, the novel synchronizer system will automatically bring the slave speed into equality with the master speed, i.e., will act on the slave governor until the frequencies of the pulse trains from differentiators 77, 78 are equal. But when such speed matching is effected, the relative phase'of the engine shafts (or that of the two pulse trains) may have almost any value. It is often desirable that further minor adjustments be effected until the phase angle is trimmed to a particular value. FIG. 7 illustrates a simple, manually adjusted phase controller associated with a masterslave type digital synchronizing system embodying the present invention. In the particular embodiment shown, the slave engine may be equipped with either a polarized" or non-polarized magnetic ball head speed governor. If, however, a non-polarized governor is employed, a magnetic reference level for the governor must be established (as described above with reference to FIG. 1) while the speeds of the master and slave engines are being roughly balanced by manipulation of their control levers 30 and 31.

As shown in FIG. 7, the governor coil 15] is connected for excitation from an operational amplifier operating from positive and negative supply voltage sources, so that exciting current may flow in either a tions in the duty ratio of the digital control signal. Specifically, taking a very simple example for illustrative purposes, if the duty ratio of the digital control signal is relatively small, say 20 percent or so, the current flow through the governor coil 151 will be in a right-to-left direction toward the negative supply source. If, on the other hand, the duty ratio of the digital control signal is relatively high, say percent or so, the current flow through the coil will be in a left-to-right direction away from the positive supply source.

The phase control is carried out by providing an adjustable reference current through the governor coil 151 and causing variations in the duty ratio of the digital control signal to increase or decrease the total effective dc. current. To accomplish that the digital control signal appearing at the set output Q of the flip-flop 79 is applied via an input resistor 152 to the inverting input of an operational amplifier 153. The noninvcrting input of the operational amplifier is coupled by a drift stabilizing resistor 154 to the slider of a potentiometer 155 which, in turn, is coupled between oppositely polarized supply sources. The operational amplifier 153 has its output coupled to the governor biasing coil 151, and between the output and inverting input of the operational amplifier there is a feedback resistor 156.

It is important that the ratio of the feedback resistor 156 to the input resistor 152 be selected to establish a sufficiently low closed loop gain for the operational amplifier 153 that saturation of that amplifier is at all times avoided. As long as that condition is met, the reference level for the digital control pulses may be varied by adjusting the setting of the potentiometer 155. For example, referring to FIG. 3 for the case where a nonpolarized governor is employed, it will be appreciated that if a fixed positive voltage is summed with the digital control signal supplied by the flip-flop 79, the same average coil current as is required to hold the slave engine in synchronism with the master will be supplied at a decreased digital control signal duty ratio. Thus, the phase angle between the synchronized master and slave engines may be adjusted by manipulation of the setting of the potentiometer 155 and the synchronizing system will then maintain the selected phase angle.

F. Digital Synchronizing With Automatic Phase Control FIGS. 8 and 9 differ from FIG. 7 in showing the organization and operation of a system which controls and adjusts phasing automatically. Specifically, for automatic phase control, provision is made to compare the integral of the digital control signal supplied by the flipflop 79 against a reference voltage. To that end, in the illustrated embodiment, there is a second operational amplifier 161 which has its inverting input coupled to the set output Q of the flip-flop 79 by an input resistor 162 and its non-inverting'input coupled to the slider of the potentiometer 155 by a drift stabilizing resistor 163. The output of the second operational amplifier 161 is, in turn, connected to the non-inverting input of the first operational amplifier 153. Further, connected between the output and inverting input of the operational amplifier 161 is a feedback capacitor 164.

As will be appreciated, the operational amplifier 161 integrates or averages the digital control signal supplied by the flip-flop 79 with a time constant which is primarily determined by the values of the resistor 162 and the feedback capacitor 164. Thus, the reference voltage applied to the non-inverting input of the operational amplifier 153 varies as a time-integral function of the difference between (a) the average value of the digital control signal, -'-which is proportional to its duty ratio, and (b) the value of the reference voltage supplied by the potentiometer 155. Whenever the system is operating with that difference other than zero, the output of amplifier 161 will increase or decrease to change the reference level at the non'inverting input of amplifier 153, thereby trimming the magnetic bias to the slave governor until the difference is restored to zero. Consequently, there is one, and only one, phase angle between the master and slave engines at which synchronism may be achieved for any given setting of the potentiometer 155. For example, say that the slave engine is equipped with a non-polarized governor and that it is initially fast relative to the master engine. In that event, as shown in FIG. 9, a synchronous condition may be initially approached at some time t -t but the synchronizing system will not then stabilize unless the master and slave engines are in their desired or set point phase relationship. Instead, the average coil current (dot-dashed line) will continue to fall because the phase reference voltage applied to the non-inverting input of the amplifier 153 reduces the effective magnitude (shaded area) of the digital control signal. The reduction in the effective amplitude of the digital control signal follows from the fact that reference for the digital control signals is offset from the reference for the magnetic biasing coil by the phase reference voltage. Ultimately, at some later time t:,, the desired or set point phase relationship will be established between the master and slave engines and a stable condition of synchronism will be achieved since the effective digital control signal is then sufficient to hold the average coil current substantially constant.

To insure that the system remains stable despite the averaging or integrating action of both the slave engine governor and the operational amplifier 161, it is recommended that the values of the resistor 162 and the capacitor 164 be selected so that the time constant for the integration performed by the operational amplifier 161 is relatively long, say, ten times longer than the time constant of the governor.

G. Synchronizing With Non-Polarized" Magnetic Ball Head Speed Governors Magnetically Referenced to Zero i Finally, if only two engines are to be synchronized, each may be equipped with a respective nonpolarized" magnetic ball head speed governor. In that event, the engines may be automatically synchronized in accordance with this invention without first establishing a magnetic bias reference level on either one of the governors. In other words, the non-polarized magnetic ball head speed governors may be magnetically referenced to zero, with the attendant advantages of eliminating the need for the operator to manually initiate the automatic synchronizing process of reducing the riskof reference drift. 1

More particularly, as shown in FIGS. and 11, this embodiment of the invention is very similar to the embodiment previously described in connection with FIGS. 1, 3 and 4. The principal exception is that the engines 21 and 22 are not operated as master and slave, but instead each of them is equipped with a respective non-polarized" speed governor. Further, the magnetic biasing coils 57' and 57" of the governors are coupled to the collectors of respective transistors 59a and 59b which, in turn, have their bases respectively coupled by current limiting resistors 82a and 82b to the set and reset outputs Q and Q of the flip-flop circuit 201. The digital control signals appearing at the set and reset outputs of the flip-flop 201 have their duty ratios complementarily modulated in accordance with any difference between the engine speeds, as previously described.

The transistors 59a and 59b alternately switch be tween their states of saturated conduction and nonconduction as the flip-flop 201 is set and reset. Specifically, as the flip-flop is switched to its set state, the transistor 59b is switched into saturation by current flow through the current limiting resistor 82b and the transistor 59a is switched out of conduction. Contrariwise, as the flip-flop is reset, the transistor 59b is switched out of conduction while the transistor 59a switches into saturation.

The digital control signals applied to the magnetic biasing coils 55a and 55b are averaged by the electrical and mechanical inertias of the respective governors. Thus, as shown by the dot-dashed lines in FIG. 11, the average current flow through the coil of each governor varies as a function of any difference between the speeds of the two engines. Moreover, since a zero magnetic reference level is employed, the current 'flow through the governor coils causes the speeds of the engines to increase somewhat from the speedsat which they would run if the governors operated at the set point speeds called for solely by the adjustments of their speeder springs. The incremental increase in the engine speeds is not, however, identical unless the speed spring settings for the two engines happen to be equal. Instead, the speed of the slower engine is increased more than the speed of the faster engine by an amount sufficient to permit synchronism to be achieved.

More particularly, any difference between the speeds of the engines causes the duty ratio of the digital control signal for the slower engine to increase as the duty ratio of the digital control signal for the faster engine decreases. For example, as shown in FIG. 11, if the engine 21 is slow relative to the engine 22, the repetition rate of the set pulses from the differentiator 77 is lower than the repetition rate of the reset pulses from the differentiator 78. Consequently, the time interval between successive set and reset pulses for the flip-flop 201 is progressively decreased, thereby causing the duty ratio of the control signal for the engine21 to increase as the duty ratio of the control signal for the engine 22 decreases. Thus, the average current flow through the coil 57' of the governor for the slower engine 21 is ultimately high relative to the average current flow through the coil 57" of the governor for the faster engine 22. Hence, there is a difference between the magnetic bias levels applied to the flyweights of the respec tive governors such that speed synchronism is achieved. The effective set point speed, at which both engines will run in synchronism, may be adjusted by changing the setting for the speeder spring in either one of the two governors.

CONCLUSION As will now be understood, the present invention provides a digital system for smoothly and continuously synchronizing prime movers. The systems carry out the means has first and second outputs for supplying first and second digital control signals having duty cycles complementarily modulated in accordance with said speed difference; and further including a bridge circuit having one set of opposed terminals for coupling said bridge across a supply source, another set of opposed terminals coupled across said coil, first and second pairs of diagonally opposed arms, a respective switch means in each of said arms, means responsive to said first control signal for selectively switching the switch means in said first pair of arms into conduction, and means responsive to said second control signal for selectively switching the switch means in said second pair of arms into conduction, whereby first and second paths for current flow through said coil in opposite directions are alternately completed so that the magnetic bias is varied in a sense dependent on the sense of said speed difference.

synchronizing control directly without the use of mechanical linkages and do not have any appreciable dead band. Hence, they detect and eliminate even very slight differences between the speeds of the prime movers. Moreover, it will be appreciated that manual or automatic phase controllers may be combined with some embodiments of the digital synchronizing system thereby permitting a desired phase relationship to be established and maintained between the synchronized prime movers.

I claim: 1. A system for controlling the speed of a prime mover comprising the combination of a magnetic ball head speed governor, including a magnetic biasing coil, for regulating the speed of said prime mover;

reference means for providing a first electrical signal having a frequency proportional to a desired speed for said prime mover;

transducer means for monitoring the actual speed of said prime mover to provide a second electrical signal having a frequency proportional to the actual speed; and

bistable means having one input coupled to said reference means, another input coupled to said transducer means and an output coupled to the biasing coil of said governor for applying a digital control signal having a duty cycle modulated in accordance with any difference between said actual and desired speeds to said biasing coil to thereby correctively adjust the speed of said prime mover for operation at said desired speed.

2. The control system of claim 1 wherein said gover- 3. The control system of claim 1 wherein said governor is polarized.

4. The control system of claim 3 wherein said bistable 5. The control system of claim 1 wherein said goverwhich varies relatively rapidly, whereby the magnetic bias on said governor is varied by said dc. component for correctively adjusting the prime mover speed and by said oscillating component to reduce the effects of any friction within the governor.

6. A system for synchronizing the speeds of prime movers, said system comprising the combination of speed regulating means for at least one of said prime movers, said regulating means including a magnetic ball head speed governor having a magnetic biasing coil;

first and second transducer means for respectively monitoring the speeds of said one prime mover and another of said prime movers to provide first and second signals having frequencies respectively proportional to the speeds of said one and said other prime movers;

bistable means having a first input coupled to said first transducer and a second input coupled to the other of said transducer for providing a digital control signal having a duty cycle modulated in accordance with any difference between the speeds of said one and said other engines; and

means coupled between the bistable means and the magnetic biasing coil of said governor for applying said digital control signal to said coil, whereby the magnetic bias on said governor is varied in response to any speed difference to thereby correctively adjust the speed of said one prime mover in a manner tending to eliminate the speed difference.

7. The synchronizing system of claim 6 wherein said governor is non-polarized, and further including a bias resistor, means for coupling said coil in series with a supply source, and switch means in series with said coil for initially coupling said coil in series with said bias resistor to thereby establish a predetermined, non-zero magnetic bias reference level for said governor and for then coupling said coil to said bistable means.

8. The synchronizing system of claim 6 wherein said governor is polarized, and said bistable means has first and second outputs for supplying first and second digital control signals having duty cycles complementarily modulated in accordance with said speed difference; and further including a bridge circuit having one set of opposed terminals for coupling said bridge across a supply source, another set of opposed terminals coupled across said coil, first and second pairs of diagonally opposed arms, a respective switch means in each of said arms, means responsive to said first control signal for selectively switching the switch means in said first pair of arms into conduction, and means responsive to said second control signal for selectively switching the switch means in said second pair of arms into conduction, whereby first and second paths for current flow through said coil in opposite directions are alternately. completed so that the magnetic bias is varied in a sense dependent on the sense of said speed difference.

9. The synchronizing system of claim 6 wherein said governor has a time constant selected so that the coil is energized in response to the digitaleontrol signal applied thereto by a current having a dc. component which varies as a function of any duty cycle modulation of said control signal and a superimposed oscillating current which varies relatively rapidly, whereby the magnetic bias on said governor is varied by said dc.

, component for correctively adjusting the prime mover speed and by said oscillating component to reduce the effects of any friction within the governor.

10. A system for synchronizing the speeds of first and second prime movers comprising the combination of first and second magnetic ball head speed governors,

each including a respective magnetic biasing coil, for respectively regulating the speeds of said first and second prime movers;

first and second transducers for respectively monitoring the speeds of said first and second prime movers to provide first and second electrical signals having frequencies respectively proportional to the speeds of said first and second prime movers; and bistable means having one input coupled to said first transducer, another input coupled to said second transducer, one output coupled to the coil of said first governor and another output coupled to the coil of said second governor, whereby said bistable means provides first and second digital control signals having duty cycles complementarily modulated in accordance with any difference between the speeds of said prime movers for differentially varying the magnetic biases on said governor in a manner tending to eliminatethe speed difference. 11. The synchronizing system of claim wherein the governor for each prime mover has a time constant selected so that the coil is energized in response to the digital control signal applied thereto by a current having a dc. component which varies as a function of any duty cycle modulation of said control signal and a superimposed oscillating current which varies relatively rapidly, whereby the magnetic bias on said governor is varied by said dc. component for correctively adjusting the prime mover speed and by said oscillating component to reduce the effects of any friction within the governor.

12. A system for synchronizing the speeds of and controlling the phase angle between first and second rotating prime movers, said system comprising the combination of a magnetic ball head speed governor, including a magnetic biasing coil, for regulating the speed of the first prime mover; f

first and second transducer means for monitoring the speeds of said prime movers to provide first and second electrical signals having frequenciesrespeo tively proportional to the speeds of said first and second prime movers;

bistable means having one input coupled to said first transducer and another input coupled to said second transducer for providing a digital control signal having a duty cycle modulated in accordance with any difference between the speeds of said prime movers;

means for supplying a phase reference voltage;

means coupled to said bistable means and said phase reference means for algebraically summing said phase reference voltage and said digital control signal to supply a sum signal;

and means for applying said sum signal to energize the coil of said governor.

13. The synchronizing and phasing system of claim 12 wherein said phase reference voltage is provided by a manually variable means, whereby the phase angle between said prime movers is manually controllable.

14. The synchronizing and phasing system of claim 13 wherein said phase reference voltage and said digital control signal are summed by a non-saturating operational amplifier having an output coupled to said biasing coil.

15. The synchronizing and phasing system of claim 14 wherein said governor is non-polarized and has a predetermined, non-zero magnetic bias reference level.

16. The synchronizing and phasing system of claim 12 wherein said phase reference voltage is provided by an integrator means having an input coupled to said bistable means and an output coupled to said summing means for supplying a voltage dependent on a time averaged level of said digital control signal as said phase reference voltage.

17. The synchronizing and phasing system of claim 16 wherein said governor and said integrator means have respective time constants; the time constant of said integrator means being selected to be long relative to the time constant of said governor; and the time constant of said governor being selected so that said coil is energized by a current having a dc. component, which varies as a function of the duty cycle of said digital control signal and variations of said phase reference voltage to correctively adjust the speed of said first prime mover and the phase angle between it and said second prime mover, and a superimposed oscillating dither signal component which varies relatively rapidly.

18. The synchronizing and phasing system of claim 16 wherein said phase reference voltage and said digital control signal are summed by a non-saturating operational amplifier having an output coupled to said biasing coil. l v i 19. The synchronizing and phasing system of claim 18 wherein said governor is non-polarized, and said reference voltage further includes a voltage for establishing a non-zero magnetic bias reference level for said governor. 

1. A system for controlling the speed of a prime mover comprising the combination of a magnetic ball head speed governor, including a magnetic biasing coil, for regulating the speed of said prime mover; reference means for providing a first electrical sIgnal having a frequency proportional to a desired speed for said prime mover; transducer means for monitoring the actual speed of said prime mover to provide a second electrical signal having a frequency proportional to the actual speed; and bistable means having one input coupled to said reference means, another input coupled to said transducer means and an output coupled to the biasing coil of said governor for applying a digital control signal having a duty cycle modulated in accordance with any difference between said actual and desired speeds to said biasing coil to thereby correctively adjust the speed of said prime mover for operation at said desired speed.
 2. The control system of claim 1 wherein said governor is non-polarized, and further including a bias resistor, means for coupling said coil in series with a supply source, and switch means in series with said coil for initially coupling said coil in series with said bias resistor to thereby establish a predetermined, non-zero magnetic bias reference level for said governor and for then coupling said coil to said bistable means.
 3. The control system of claim 1 wherein said governor is polarized.
 4. The control system of claim 3 wherein said bistable means has first and second outputs for supplying first and second digital control signals having duty cycles complementarily modulated in accordance with said speed difference; and further including a bridge circuit having one set of opposed terminals for coupling said bridge across a supply source, another set of opposed terminals coupled across said coil, first and second pairs of diagonally opposed arms, a respective switch means in each of said arms, means responsive to said first control signal for selectively switching the switch means in said first pair of arms into conduction, and means responsive to said second control signal for selectively switching the switch means in said second pair of arms into conduction, whereby first and second paths for current flow through said coil in opposite directions are alternately completed so that the magnetic bias is varied in a sense dependent on the sense of said speed difference.
 5. The control system of claim 1 wherein said governor has a time constant selected so that the coil is energized in response to the digital control signal applied thereto by a current having a dc. component which varies as a function of any duty cycle modulation of said control signal and a superimposed oscillating current which varies relatively rapidly, whereby the magnetic bias on said governor is varied by said dc. component for correctively adjusting the prime mover speed and by said oscillating component to reduce the effects of any friction within the governor.
 6. A system for synchronizing the speeds of prime movers, said system comprising the combination of speed regulating means for at least one of said prime movers, said regulating means including a magnetic ball head speed governor having a magnetic biasing coil; first and second transducer means for respectively monitoring the speeds of said one prime mover and another of said prime movers to provide first and second signals having frequencies respectively proportional to the speeds of said one and said other prime movers; bistable means having a first input coupled to said first transducer and a second input coupled to the other of said transducer for providing a digital control signal having a duty cycle modulated in accordance with any difference between the speeds of said one and said other engines; and means coupled between the bistable means and the magnetic biasing coil of said governor for applying said digital control signal to said coil, whereby the magnetic bias on said governor is varied in response to any speed difference to thereby correctively adjust the speed of said one prime mover in a manner tending to eliminate the speed difference.
 7. The synchronizing system of claim 6 wherein said governor is non-polarized, and furtheR including a bias resistor, means for coupling said coil in series with a supply source, and switch means in series with said coil for initially coupling said coil in series with said bias resistor to thereby establish a predetermined, non-zero magnetic bias reference level for said governor and for then coupling said coil to said bistable means.
 8. The synchronizing system of claim 6 wherein said governor is polarized, and said bistable means has first and second outputs for supplying first and second digital control signals having duty cycles complementarily modulated in accordance with said speed difference; and further including a bridge circuit having one set of opposed terminals for coupling said bridge across a supply source, another set of opposed terminals coupled across said coil, first and second pairs of diagonally opposed arms, a respective switch means in each of said arms, means responsive to said first control signal for selectively switching the switch means in said first pair of arms into conduction, and means responsive to said second control signal for selectively switching the switch means in said second pair of arms into conduction, whereby first and second paths for current flow through said coil in opposite directions are alternately completed so that the magnetic bias is varied in a sense dependent on the sense of said speed difference.
 9. The synchronizing system of claim 6 wherein said governor has a time constant selected so that the coil is energized in response to the digital control signal applied thereto by a current having a dc. component which varies as a function of any duty cycle modulation of said control signal and a superimposed oscillating current which varies relatively rapidly, whereby the magnetic bias on said governor is varied by said dc. component for correctively adjusting the prime mover speed and by said oscillating component to reduce the effects of any friction within the governor.
 10. A system for synchronizing the speeds of first and second prime movers comprising the combination of first and second magnetic ball head speed governors, each including a respective magnetic biasing coil, for respectively regulating the speeds of said first and second prime movers; first and second transducers for respectively monitoring the speeds of said first and second prime movers to provide first and second electrical signals having frequencies respectively proportional to the speeds of said first and second prime movers; and bistable means having one input coupled to said first transducer, another input coupled to said second transducer, one output coupled to the coil of said first governor and another output coupled to the coil of said second governor, whereby said bistable means provides first and second digital control signals having duty cycles complementarily modulated in accordance with any difference between the speeds of said prime movers for differentially varying the magnetic biases on said governor in a manner tending to eliminate the speed difference.
 11. The synchronizing system of claim 10 wherein the governor for each prime mover has a time constant selected so that the coil is energized in response to the digital control signal applied thereto by a current having a dc. component which varies as a function of any duty cycle modulation of said control signal and a superimposed oscillating current which varies relatively rapidly, whereby the magnetic bias on said governor is varied by said dc. component for correctively adjusting the prime mover speed and by said oscillating component to reduce the effects of any friction within the governor.
 12. A system for synchronizing the speeds of and controlling the phase angle between first and second rotating prime movers, said system comprising the combination of a magnetic ball head speed governor, including a magnetic biasing coil, for regulating the speed of the first prime mover; first and second transducer means for monitoring the speeDs of said prime movers to provide first and second electrical signals having frequencies respectively proportional to the speeds of said first and second prime movers; bistable means having one input coupled to said first transducer and another input coupled to said second transducer for providing a digital control signal having a duty cycle modulated in accordance with any difference between the speeds of said prime movers; means for supplying a phase reference voltage; means coupled to said bistable means and said phase reference means for algebraically summing said phase reference voltage and said digital control signal to supply a sum signal; and means for applying said sum signal to energize the coil of said governor.
 13. The synchronizing and phasing system of claim 12 wherein said phase reference voltage is provided by a manually variable means, whereby the phase angle between said prime movers is manually controllable.
 14. The synchronizing and phasing system of claim 13 wherein said phase reference voltage and said digital control signal are summed by a non-saturating operational amplifier having an output coupled to said biasing coil.
 15. The synchronizing and phasing system of claim 14 wherein said governor is non-polarized and has a predetermined, non-zero magnetic bias reference level.
 16. The synchronizing and phasing system of claim 12 wherein said phase reference voltage is provided by an integrator means having an input coupled to said bistable means and an output coupled to said summing means for supplying a voltage dependent on a time averaged level of said digital control signal as said phase reference voltage.
 17. The synchronizing and phasing system of claim 16 wherein said governor and said integrator means have respective time constants; the time constant of said integrator means being selected to be long relative to the time constant of said governor; and the time constant of said governor being selected so that said coil is energized by a current having a dc. component, which varies as a function of the duty cycle of said digital control signal and variations of said phase reference voltage to correctively adjust the speed of said first prime mover and the phase angle between it and said second prime mover, and a superimposed oscillating dither signal component which varies relatively rapidly.
 18. The synchronizing and phasing system of claim 16 wherein said phase reference voltage and said digital control signal are summed by a non-saturating operational amplifier having an output coupled to said biasing coil.
 19. The synchronizing and phasing system of claim 18 wherein said governor is non-polarized, and said reference voltage further includes a voltage for establishing a non-zero magnetic bias reference level for said governor. 